Hi, I had the same problem and solved with this change in /etc/php/7.4/fpm/pool.d/librenms.conf file:
user = librenms
group = librenms
listen = /run/php/php-fpm-librenms.sock
;listen = /run/php/php7.4-fpm.sock
; Set listen(2) backlog.
; Default Value: 511 (-1 on FreeBSD and OpenBSD)
;listen.backlog = 511
; Set permissions for unix socket, if one is used. In Linux, read/write
; permissions must be set in order to allow connections from a web server. Many
; BSD-derived systems allow connections regardless of permissions.
; Default Values: user and group are set as the running user
; mode is set to 0660
listen.owner = www-data
listen.group = www-data
;listen.mode = 0660
Also in /etc/nginx/conf.d/librenms.conf change line 14 into this:
fastcgi_pass unix:/run/php/php-fpm-librenms.sock;
Restart both services and it should work.